The present invention relates to an electro-pneumatic positioner for use with a pneumatically controlled two-chamber actuator comprising a first valve unit comprising a first air supply valve and a first air outlet valve connected to a conduit, which in turn is connected to a chamber of the actuator, a second valve unit comprising a second air supply valve and a second air outlet valve connected to a conduit, which in turn is connected to a second chamber of the actuator; a first control device acting on the first air supply valve, which first control device comprises a first control valve; a second control device acting on the said second air supply valve, which second control device comprises a second control valve.
Positioners are widely used in many applications, such as controlling the opening degree of valves in different processes. With these pressurised fluid is controlled and directed to the chambers of an actuator, often in the form of a cylinder/piston arrangement.
In recent years so called electro-pneumatic positioners have come in use. These often comprise piezo-electrical valves controlling the flow of pressurised air through the positioner in response to electrical signals to the piezo-electrical valves.
One type of electro-pneumatic positioner is described in patent document DE 196 35 368. The positioner comprises two main valve units, each comprising a main air supply valve and a main air outlet valve. Each unit is in communication with a respective chamber of an actuator. Each valve unit is controlled by a piezo-electrical control valve acting on a switch valve in the main air supply valve. As shown in the drawing of the patent, air via the control valve acts directly on the switch valve.
Further in order to have a simplified function of the positioner, a type of cross-connection is made, whereby the main outlet air valve of the first valve unit is controlled by air taken from the switch valve of the further main air supply valve and that the main outlet air valve of the further valve unit is taken from the switch valve of the first main air supply valve. With this cross-connection only two piezo-electrical valves are required to control two supply valves and two outlet valves, thus simplifying the overall design and reducing the manufacturing costs.
One major drawback with the solution according to DE 196 35 368 is that, as mentioned above, the piezo-electrical valve acts directly on the membrane of the air supply valve. Since the air pressure of the air supply is small, in the order of 1-1.5 bar, and the air orifices in the piezo valve are rather small in order for the valve to be able to close, the air flow will consequently be small, typically in the order of 1.5 1/min. This rather small air flow acting on the membrane of the air supply valve gives rise to a slow reaction of the supply valve in response to an opening of the piezo-electrical valve. This also puts a restraint on the size of the valve openings in the air supply valve.
Another drawback with the positioner according to DE 196 35 368 is that the pressurised air for controlling the air outlet valves is taken from the outlet of the air supply valves. This means in practice that there probably is a certain delay in the system. If for instance the positioner is to activate one side of the actuator, one of the piezo-electrical valves opens and provides air to the air supply valve of one of the valve units, which in turn opens a passage for the process air supply, e. g. an air compressor, which is led to one chamber of the actuator. This process air supply is further led to the air outlet valve of the other valve unit. Since there is a certain sequential activation of the valves a delay might be obtained.
Another disadvantage with working directly with the process air is that the pressure is known to fluctuate somewhat. Due to these fluctuations in pressure the controlling force on the air outlet valves fluctuate also, which means unpredictable variations in control characteristics.
The aim of the present invention is to provide a positioner which remedies the above problems, which provides a positioner with better performance and faster and more reliable control characteristics.
In this context it is to be understood that the word positioner comprises both positioners able of positioning an actuator at any position along its stroke as well as positioning an actuator at its end positions, i e a sort of on-off function.
According to one aspect of the invention this aim is obtained by a positioner according to the preamble of patent claim 1 characterised in that said first control device further comprises a first air supply means able of supplying pressurised air to said first air supply valve and that said first control valve acts on and controls said first air supply means; that said second control device further comprises a second air supply means able of supplying pressurised air to said second air supply valve and that said second control valve acts on and controls said second air supply means; that said first air supply means is able of supplying air to said second air outlet valve, and that said second air supply means is able of supplying air to said first air outlet valve. According to another aspect of the invention, it is characterised in that the air flow rate that the air supply means are able of supplying are substantially larger than the air flow rate that the control valve is able of supplying in order to control the air supply means.
There are several benefits with the invention. By using an air supply means controlled by the control valve, much larger air flow rates are obtainable for controlling the valves of the positioner, than with positioners according to the state of the art, where the air flow from the control valve was used directly to control the valves of the positioner. That becomes especially noticeable with piezo-electrical control valves, which only can cope with airflow rates in the order of 0.5 1/min. One of the benefits with a larger air flow rate is that the response time for the positioner valves is greatly reduced, providing a quicker reaction of the positioner upon a control signal.
Further with the larger air flow rates, the power obtainable for controlling the valves of the positioner is increased, i e larger membrane and piston areas may be used, thereby providing for the possibility of having larger openings in the valves with maintained valve performance and characteristics. These larger openings reduce the risk of particles in the pressurised air depositing inside the valve and disrupting the function.
Preferably the pressurised air that the air supply means obtains from the air supply source has passed a pressure regulator valve. In doing this a predictable pressure level is obtained for controlling the valves, which is important in order to achieve reliable and predictable characteristics of the positioner and thus the actuator.
These and other aspects of, and advantages with, the present invention will become apparent from the following detailed description of a preferred embodiment of the invention with reference to the accompanying drawings.